1. Field of the Invention
The present invention relates to a method and a device for measuring the content of chemicals in connection with bleaching carried out with the use of bleaching agents, preferably cellulose fibres in a pulp suspension. The object is to provide a better and more uniform product quality and to prevent overdosing of the bleaching chemical used.
2. Brief Description of the Related Art
In the pulp and paper industry, the bleaching of cellulose fibres is essential to the final product quality. Principally, the bleaching has two aims, namely first to create a continuation of the lignin-separating process, which consists of cooking in the manufacture of chemical pulps, and second to remove the dark colour given to the pulp by certain organic substances so that a certain brightness is created. Previously, chemicals based on chlorine were extensively used as bleaching agents. However, law changes, partly triggered by massive public opinion concerning influence on the environment, have enforced chlorine-free bleaching processes, in which context e.g., peroxide bleaching has been used to an increasing extent both for chemical and mechanical pulp as well as for recycled fibres. Peroxide is a relatively expensive chemical, for which reason a better control of the bleaching process would mean substantial savings in terms of chemical consumption and, at the same time, provide a better and more uniform product quality. To achieve better control it is necessary to measure the content of peroxide in the pulp after the bleaching, the so-called residual peroxide. A condition for a good bleaching result is that there always exists a certain content of residual peroxide, the amount of which in turn depends on the type of pulp and the type of bleaching process. On the other hand, too high a content of residual peroxide means economical losses since it is caused by an overdosing of the bleaching chemical, or bleaching agent, used in the process.
The content of residual peroxide is measured for the reason that the content of bleaching agent can vary from a few ppm up to several thousand ppm. The content is usually determined in the laboratory by means of a titration method. To carry out the analysis in a laboratory is normally not advisable in connection with automatic process control because the results are not available after a sufficiently short delay and also because the laboratory analysis requires much staff work. Better supervision of, e.g., the residual peroxide content accordingly requires access to an instrument for continuous (on-line) measurements during the process. The necessary measurement frequency may, depending on the bleaching process used, vary between approximately 3 to 10 minutes. The measurement is then carried out on a filtration sample from the bleached pulp suspension, the concentration of which may be 15% and higher. The sample can be derived either by means of a special sampler, extracting a fibre-free sample from the pulp suspension, or directly out of the reject from a press which can be located after the bleaching tower. The properties of the filtrate vary with the bleaching process and with the sampling position. Normally, peroxide bleaching is carried out at a pH value of 8.5-11.5 and at a temperature between 850 and 120.degree. C. (lower temperatures are to be used for recycled pulp). The filtrate can contain, inter alia, chemicals used to stabilize peroxide and complex binding of metal ions and to adjust the pH value. In addition thereto, there can also be found in the filtrate suspended material in the form of fine fibre material, filling agents (clay, chalk . . . ), collodial lignin, printing inks, etc.
The instruments today used in the pulp industry for on-line measurement of the residual peroxide content are generally based on the titration principle or on electro-chemical methods. However, those instruments, for various reasons, do not operate satisfactorily. From a result point of view, automatic titration instruments perform rather well but the measurement rate is much too slow, up to 30 minutes between results and, in addition thereto, they require various chemicals, which must be handled with great care and which generate waste products detrimental to the environment. Further, the instruments also require much maintenance, which naturally is a drawback in connection with continuous operation. Instruments based on electrochemical methods, such as polarography or voltametry, have low selectivity when used together with bleaching filtrates derived from pulp from the forest industry, because such filtrates have a complex composition resulting in unsatisfactory accuracy and reproducibility. In addition thereto, those instruments are relatively insensitive at low residual peroxide contents.